- Parasitic drain: Ensure there is no power leakage when the car is off. Disconnect the negative terminal overnight and reconnect it in the morning. If there is a drop of more than 50-100 mV, there might be a parasitic drain; check for abnormal loads on the battery.
- Faulty alternator: Even though the alternator tests fine, it might not consistently charge. Test the alternator's output during start-up, idle, and high RPMs.
- Battery capacity: Batteries degrade over time. Check the battery's actual capacity compared to its original rating. Consider replacing it if it's below 70-80% of its rated capacity.
- Ground/wiring issue: Ensure all ground connections and wiring are clean, corrosion-free, and tight. A bad ground connection can cause battery drain.
- Defective diode in the alternator: If the diodes are faulty, the battery may continue to discharge when the engine is not running.
- On-board electronics: Check if any interior lights or modules remain powered even after turning off the car. Faulty electronics can draw power from the battery.
